The dinoflagellates should die if exposed long enough to fresh water. I don't know what the time limit might be, though, and I'd expect a few to survive here and there. Rinsing the sand might remove some detritus. I'd be tempted, though, to soak the sand in RO/DI with a powerhead and check the phosphate level after twelve hours or so, to see whether the sand is leaching a significant amount of phosphate.